This invention relates to a method for forming a structure comprising ceramic elements bonded by a sintered silver layer. More particularly, this invention relates to a method for bonding ceramic elements that includes applying a thick film of a paste comprising a silver powder to the faying surfaces, and sintering the silver powder to produce an integral layer bonding the surfaces.
In a radio transmitting or receiving device, a stripline filter is employed to select signals within a relatively narrow frequency band, referred to as a passband, and to shunt signals having frequencies outside the passband. The stripline filter comprises a metallic resonator sandwiched between ceramic elements and capacitively coupled to an external ground plate for shunting the unwanted signals.
One method for manufacturing a stripline filter comprises soldering silver films applied to surfaces of preformed ceramic elements. The silver films are applied in a superposable pattern corresponding to the desired configuration of the resonator. The silver films are formed by applying a paste composed of silver powder dispersed in a volatile solvent containing an organic binder. The paste is dried to remove the solvent and fired to vaporize the binder and to sinter the silver particles into an integral layer.
To bond the elements together, a film of a solder paste is applied to one of the sintered silver films. The solder paste comprises particles composed of a low melting solder alloy and dispersed in a solvent vehicle, similar to the silver paste. A common solder for this purpose is composed of a tin-base alloy containing about 5 weight percent silver. The elements are arranged with the second silver film resting against the solder particulate film. The assembly is then heated at a temperature, typically less than about 300.degree. C., sufficient to melt the solder alloy and fuse the solder to the adjacent silver films. Upon cooling, the solder resolidifies and bonds to the silver films, thereby joining the ceramic elements into an integral structure.
Upon soldering, the molten solder wets the silver films, which is essential to forming a strong solder bond. In contrast, molten solder does not wet ceramic surface, and does not bond directly to the ceramic element, thus necessitating the silver films. The resulting metal layer thus comprises silver films bonded to the ceramic and solder-bonded to an intermediate reflowed tin-base alloy film.
In bonding the ceramic elements utilizing an intermediate solder film, additional steps are required to apply to solder paste, thereby adding to the cost of the product. Oxides and other contaminants on the silver surface inhibit wetting by the solder alloy and thus interfere with the formation of a strong solder bond. Furthermore, silver is soluble in the molten solder alloy. Thus, there is a tendency for silver to leach from the film into the solder alloy. Accordingly, it is necessary to carefully control the reflow time and temperature to avoid complete dissolution of the silver film that would otherwise prevent bonding to the ceramic. Still further, subsequent processing steps are limited by the solder reflow temperature, since higher temperatures are accompanied by a melting of the solder alloy, which would allow the elements to shift or cause further leaching of the silver film.